Brachythecium rivulare
Brachythecium albicans
Brachythecium moss, river ragged-moss
Brachythecium moss, lawn moss, lawn ragged moss, whitish feather-moss
leaves erect-appressed, often rigidly spreading in sympodial stems, closely imbricate, broadly ovate, broadest at 1/7 leaf length, concave, not plicate, slightly, or rarely moderately plicate, 1.5–2.5 × 0.8–1.5 mm;
base rounded, broadly long-decurrent;
margins plane or recurved at places, serrulate throughout (often serrate in sympodial stems);
apex gradually tapered, broadly acute, rarely short-acuminate;
costa to 50–70% leaf length, strong, terminal spine indistinct or absent;
alar cells short-rectangular, strongly enlarged, 20–50 × 15–30(–35) µm, walls thin, region abruptly delimited, large, of 8–15 × 8–15 cells, somewhat saclike, reaching from near margin 50% distance to costa;
laminal cells linear, 50–100 (–150) × 6–11 µm;
basal cells 15–35 × 11–15 µm, region in 3 rows.
leaves erect- appressed, straight or slightly falcate, closely imbricate, ovate-lanceolate, broadest at 1/7–1/5 leaf length, concave, not or slightly plicate, (1.3–)1.7–2.8 × (0.4–)0.6–1.1 mm;
base tapered, broadly or occasionally narrowly long-decurrent;
margins plane or often recurved almost throughout, entire, rarely serrulate;
apex gradually tapered to acumen, acumen long;
costa to 40–70% leaf length, slender, terminal spine absent;
alar cells subquadrate to short-rectangular, small, 15–30 × 11–15 µm, walls moderately thick, region extensive, of 10–15 × 6–8 cells;
laminal cells linear, 40–100(–125) × 5–9 µm;
basal cells 10–15 µm wide, region in 3 rows.
more spreading, ovate-lanceolate, 2–3:1;
costal terminal spine present;
alar region conspicuous or not;
laminal cells enlarged across base.
similar, smaller.
red-brown, 2–2.5 cm, rough.
reddish orange, 1–2 cm, smooth.
dioicous.
dioicous.
inclined to horizontal, red-brown or dark brown, ovate to elongate, curved, 2–2.5 mm;
annulus separating by fragments;
operculum long-conic.
inclined to horizontal, reddish orange, elongate, curved, 2 mm;
annulus separating by fragments;
operculum long-conic.
13–18(–22) µm.
dimorphic: 15–18 µm or 11–13 µm; these two types are in proportion 1:1, as tetrads have two big and two small spores.
Brachythecium rivulare
Brachythecium albicans
Brachythecium rivulare occurs in most states and provinces but is absent in the southeastern United States. Although B. rivulare was reported from California, no collection was confirmed (many specimens proved to be atypical phenotypes of B. frigidum). The plants form whitish green soft mats near spring water but become rigid and olive brownish when affected by strong currents. In all cases, B. rivulare is somewhat dendroid and distinguished by the peculiar alar group that is somewhat saclike, composed of strongly enlarged thin-walled cells reaching to the margins; in B. frigidum and B. rutabulum, that may consist of a fairly large pellucid group in leaf corners, but the region never reaches the margins where cells are relatively narrow and rectangular in several rows and often rich in chloroplasts. This character is sometimes difficult to find in phenotypes that have extensive sympodial branching and are also regularly pinnate.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Despite numerous records and misidentified herbarium collections from many of the states, Brachythecium albicans is common only in the Pacific coast region and the mountains of the West. The species is distinguished by the pale color, julaceous foliage, filiform and somewhat secund acumina, entire leaf margins that are often recurved along most of the leaf length, and elongate alar region. This species is variable, often producing very thin shoots, with stem leaves as small as 1.3 × 0.4 mm, but usually such collections include some larger shoots that imply more typical B. albicans. Branches of specimens from California can be as long as 18 mm. Differentiation from the Arctic 15. B. coruscum is discussed under that species. The name B. albicans is used, despite preliminary molecular data that demonstrate differences between American and northeastern Asian populations from European plants; if further study proves that they are not conspecific, then the name B. pseudoalbicans should be used for at least the populations from western North America.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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